The Clean Air Act is a complex and often confusing piece of legislation, especially when it comes to confronting the challenges of global warming, which were not fully understood when the original law was passed in 1970.

At that time, the biggest risk to the environment and human health was believed to come from more "conventional" air pollutants, like sulfur dioxide and lead. But when the threat of greenhouse gases became clear, the Environmental Protection Agency needed the flexibility to combat it quickly and effectively.

That is what the EPA has tried to do multiple times in the past several years, promulgating rules regulating the emissions of carbon dioxide — by far the most common greenhouse gas — from both automobiles and stationary sources like power plants. Those actions haven't sat well with industries that will have to pay more in the short term as a result. They have challenged the agency's authority repeatedly, but the Supreme Court ruled against them in 2007 and 2011.

Following another suit by a coalition of business interests and states, the court on Monday correctly affirmed the EPA's power and responsibility to regulate greenhouse gases under the act, even though five justices voted to disallow one of the agency's regulatory tactics. The case, Utility Air Regulatory Group vs. EPA, preserves the government's ability to confront global warming while also demonstrating Congress' persistent failure to update the law to meet modern needs.

In the absence of congressional action, the EPA was left alone to deal with an impossible situation. The act targets "major sources" of pollution, initially defined as those that emitted more than 250 tons of pollutants each year. But carbon dioxide is emitted in vastly greater amounts. To keep the focus on the biggest polluters, like power plants and refineries, and avoid regulating millions of small sources, like hospitals and local businesses, the agency increased the threshold to 100,000 tons per year.

The point, as Justice Stephen Breyer argued in a partial dissent, was to adhere to the law's clear purpose and to allow the agency the flexibility to make decisions based on new information.

Justice Antonin Scalia, writing for a 5-4 majority, rejected that position, arguing that "an agency may not rewrite clear statutory terms to suit its own sense of how the statute should operate." Still, seven justices agreed that the agency could continue to regulate significant greenhouse-gas emissions from power plants that were already being regulated for emitting conventional pollutants — in other words, almost all power plants.

The Obama administration has made commendable efforts to fight global warming, most recently by issuing a proposed rule that aims to reduce carbon pollution from existing power plants 30 percent from 2005 levels by 2030. (Monday's ruling does not directly affect the proposed power plant standards ). There will always be companies that would just as soon avoid the necessary investments in cleaner energy. That's all the more reason for the EPA to use the authority it has been granted under the law.